The present invention relates to electrochromic devices, that is, devices which exhibit a reversible color change upon the application of an electric field. Specifically, the invention relates to reversible electrochromic devices incorporating cathodic and/or anodic electrochromic layers against which a polycrystalline mica layer functions as a solid electrolyte.
A number of different electrochromic materials and processes are known. One description covering various types of electrochromic devices is given by I. F. Chang in Nonemissive Electrooptic Displays, edited by A. R. Kmetz and F. K. von Willisen, pages 155-196, Plenum, New York (1976), reporting a 1975 Brown, Boveri Symposium on display technology.
Electrochromic materials may be categorized as cathodic or anodic depending upon whether they darken as the cathode or as the anode in an electrochemical cell. Electrochromic devices typically comprise, in addition to a selected electrochromic material, a solid or liquid electrolyte in contact with the electrochromic material which acts as a source or sink for protons or other mobile cations which must move into or out of the electrochromic material to cause darkening or fading.
The darkening of cathodic electrochromic materials is thought to involve the double injection of protons from a suitable electrolyte and electrons from a suitable electronic conductor into the materials under the influence of an applied electric field. U.S. Pat. Nos. 2,829,196 and 3,521,941 describe devices incorporating cathodic electrochromic compounds such as MoO.sub.3, WO.sub.3, and compounds selected from among the group of metal tungstates, molybdates, niobates, vanadates, and titanates. These patents list selected metal oxides, sulfides, fluorides, nitrides and plastics as suitable electrolytes.
Anodic materials, which are materials which darken when functioning as the anode in an electrochemical cell, are also well known. U.S. Pat. Nos. 4,191,453 and 4,258,984 disclose that hydrated iridium oxide or iridium oxyhydroxide (IrO.sub.x. H.sub.2 O) exhibits anodic electrochromic behavior. Other known anodic electrochromics include the hydrated oxides of rhodium, nickel, cobalt and chromium. For the purpose of the present description all such hydrated oxides will be referred to as oxyhydroxides irrespective of whether the water therein is present as hydroxyl or H.sub.2 O.
Electrochromic devices may be cathodic, anodic, or complementary. By a complementary device is meant a device comprising both anodic and cathodic electrochromic layers, in combination with one or more electrolyte layers, wherein both anodic and cathodic darkening of the electrochromic layers is utilized during the operation of the device. Published Japanese patent applications Nos. 56-4679 and 56-12621 disclose complementary electrochromic devices comprising an anodic electrochromic layer and a cathodic electrochromic layer separated by an electrolyte layer. In those devices darkening of the anodic and cathodic layers occurs simultaneously or sequentially when a current source is connected to the device.
The selection of a suitable electrolyte for devices such as above described remains one of the principle problems of electrochromic device design. The electrolyte should be an insulator for electron flow (electronic insulator) but a conductor for ionic flow (ionic conductor). In addition, it should allow free transfer of cations across the electrolyte-electrochromic material boundaries without interacting with the electrochromic material in a way which will degrade either material. Preferably the electrolyte may be provided as a transparent layer so that transparent devices or devices with one or more darkening layers behind the electrolyte from the viewpoint of the user can be made.
A number of different solutions to the electrolyte problem have been proposed. U.S. Pat. No. 3,712,710 suggests the use of aluminum oxide in combination with an alkali or alkaline earth metal oxide such as Na.sub.2 O, K.sub.2 O or MgO as an insulating layer material. These materials are characterized as colorless solid ionic conductors.
Fast response in WO.sub.3 -containing electrochromic devices has been obtained by replacing the solid charge-carrying component with a liquid or gel electrolyte, typically based on sulfuric acid. U.S. Pat. No. 3,708,220 describes gel electrolytes of this type, while U.S. Pat. No. 4,175,837 discloses liquid electrolytes containing selected sodium or lithium salts. Although such electrolytes impart good electrochromic performance, problems relating to the handling and containment of liquid and gel electrolytes remain. In addition, WO.sub.3 electrochromic components are attacked by acidic electrolyte materials containing water, a process which limits the lifetime of the device.
One proposed class of solid electrolytes, disclosed in U.S. Pat. No. 3,995,943, includes certain electrically insulating silver compounds permeable to silver ions. An example of such an electrolyte is Ag.sub.4 RbI.sub.5. U.S. Pat. No. 4,106,862 suggests the use of ionic conductors of the formula Na.sub.1-x Zr.sub.2 Si.sub.x P.sub.3-x 0.sub.12, wherein x ranges from 0.8 to 2.4, as electrically insulating, sodium ion-conducting electrolyte materials.
It is a principal object of the present invention to provide electrochromic devices incorporating a new solid ion-permeable electrolyte material.
It is a further object of the invention to provide new electrochromic combinations of a solid electrolyte with anodic and/or cathodic electrochromic layers which can be self-supporting and/or flexible as well as transparent or opaque.
Other objects and advantages of the invention will become apparent from the following description thereof.